The present invention relates to improvements in a wire discharge machining method and apparatus that machines a workpiece by supplying working electric power to an interpole space between a wire electrode and the workpiece and producing a discharge.
FIG. 10 is an explanatory diagram showing a conventional wire discharge machining apparatus. In FIG. 10, this wire discharge machining apparatus comprises a wire electrode 1, a workpiece 2, a wire bobbin 3, a working fluid 4, working fluid nozzles 5a and 5b as working fluid supplying means for supplying the working fluid 4 to an interpole space between the wire electrode 1 and the workpiece 2, a capstan roller 6, a pinch roller 7, an X table 8 for driving the workpiece 2 in a horizontal direction (X direction), a Y table 9 for driving the workpiece 2 in a horizontal direction (Y direction), an X-axis servo amplifier 10 for controlling a drive motor, not shown, for driving the X table 8, a Y-axis servo amplfier 11 for controlling a drive mrotor, not shown, for driving the Y table 9, working power supplying means 12, bath voltage detecting means 13, and control means 14.
The operation will be described below. The wire electrode 1 is carried between the capstan roller 6 and the pinch roller 7, and dragged to be opposed against the workpiece 2. A working electric power as discharge energy is supplied into the interpole space between the wire electrode 1 and the workpiece 2 by the working power supplying means 12, while the working fluid 4 is supplied into the interpole space through the working fluid nozzles 5a and 5b, whereby the workpiece 2 is worked into a predetermined contour shape by moving the workpiece 2 relative to the wire electrode 1 employing the X table 8 and the Y table 9 as positioning means. The control means 14 controls the positioning means to position the workpiece 2 relative to the wire electrode 1 and makes control for the electrical machining conditions.
FIG. 11 is an explanatory view showing a machining example of a corner portion by the conventional wire discharge machining apparatus. In FIG. 11, reference numeral 1 denotes a wire electrode, reference numeral 2 denotes a workpiece, reference numeral 15a denotes an outer corner, reference numeral 15b denotes an Lnner corner, and the paths A to E denote machining paths of the wire electrode 1 to machine the workpiece 2.
FIG. 11(a) is a view showing a machining example in which an edge corner portion is machined, and FIG. 11(b) is a view showing a machining example in which a circular arc corner portion is machined.
For instance, in a case where the edge corner portion of the workpiece 2 is worked by moving the wire electrode 1 along the path A to C to E, it is known that a round shear droop as indicated by the solid line is produced in the outer corner portion 15a and the inner corner portion 15b, as shown in FIG. 11(a). Thi s shear droop may be caused by low rigidity of the wire electrode 1. That is, the wire electrode 1 is deflected due to a discharge reaction force developed between the wire electrode 1 and the workpiece 2, forcing the wire electrode 1 to actually take the path A to B to D to E, so that the workpiece 1 is worked excessively in the outer corner portion 15a, and unworked in the inner corner portion 15b. The size of this shear droop is increased as the discharge reaction force is larger, or the machining rate is higher, and gets to about several tens xcexcm to several hundreds xcexcm in the ordinary roughing.
In a case where the circular arc corner portion of FIG. 11(b) is worked, the wire electrode 1 is run along the path A to B to F to D to E, but practically takes the path A to B to G to D to E, due to the same reason, so that a shear droop is produced in the outer corner portion 15a and the inner corner portion 15b in the same way as in FIG. 11(a).
As described above, in working the corner portion, there was a problem that a shear droop was produced in the edge corner portion and the circular arc corner portion, resuitino in a lower precision of the machined configuration.
The techniques for preventing such shear droop from arising in the ccrner portion of the workpiece in machining the corner portionweredisclosedin JP-A-2571077, JP-A-8-39356 and JP-A-2000-84743. The conventional art involves improvements in the machining precision of the corner portion by changing the relative movement speed of the wire electrode to the workpiece before and after the corner portion, and the electrical machining conditions to reduce a deflection of the wire electrode while machining the corner portion.
In these conventional arts, however, the corner portion is worked by using a significantly smaller discharge energy than when the linear portion is worked to reduce deflection of the wire electrode in working the corner portion, resulting in quite lower machining rate. Such a significant reduction in the machining rate may be the to be a lethal problem for the wire discharge machining apparatus in the light of the present situation in which the important factors of performance required are regarded to be the machining precision and the machining productivity.
Along with the improvements in the wire discharge machining in the respects of the power control for preventing disconnection or the wire electrode, the machining rate of the wire discharge machining is increased rapidly up to about 200 to 250 mm2/min, or about 350 to 400 mm2/min at the maximum machining rate, for example. That is, the discharge energy input into the interpole space between the wire electrode and the workpiece is increased. Accordingly, the wire electrode is more deflected during the machining due to an increased discharge reaction force. In such current situation, there is a problem that when the corner portion is worked at a desired precision employing the techniques of the conventional art, the machining rate is decreased significantly in the corner portion, offsetting the effect of higher machining rate, irrespective of an increased input discharge energy.
As described above, it is very important that the machining configuration precision of the corner portion is improved in machining the corner portion of the workpiece, and the machining productivity is enhanced by recducing the machining time for the corner portion to the minimum.
As a method of suppressing the increased machining time for the corner portion, it is known to correct the machining path. However with such method, the corner portion may be worked while the wire electrode is deflected, whereby it was impossible to improve the configuration precision of the corner portion over the entire area from the upper face of the workpiece through the sublevel face to the lower face. Accordingly, it is considered that the wire electrode may be deflected as least as possible, namely, the discharge energy may be decreased down to the minimum, at the corner portion to improve the machining configuration precision of the corner portion over the entire area from the upper face of the workpiece through the sublevel face to the lower face, thereby working the corner portion at the slowest machining rate. Hence, in the conventional art, there was another problem that the machining configuration precision and the machining productivity could not be improved consistently in working the corner portion of the workpiece.
The present invention has been achieved to solve the above-menticned problems, and it is an object of the invention to provide a wire discharge machining method and apparatus in which the machining configuration precision and machining productivity for the corner portion can be improved consistently in working the corner portion.
A wire discharge machining method according to this invention for machining a workpiece by supplying a discharge energy into an interpole space between a wire electrode and the workpiece while moving the workpiece relative to the wire electrode includes a first step of working the workpiece by decreasing gradually the discharge energy from a point located a first predetermined distance prior to a corner portion entry on a machining path of the wire electrode, a second step of stopping the relative movement between the wire electrode and the workpiece at the corner portion entry, a third step of releasing the stopped relative movement in accordance with a predetermined decision criterion, a fourth step of working the workpiece in the corner portion and a portion extending a second distance away from the corner portion on the machining path of the wire electrode with a lower discharge energy that is reduced at a certain reduction ratio to the discharge energy at the first predetermined distance prior to the corner portion entry according to at least one of the first condition that is determined by the corner configuration, such as the radius and angle of the corner portion and the second condition that is determinedby the requisition specification such as a desired machining configuration precision, and a fifth step of working the workpiece by increasing gradually the discharge energy after passing the second distance till passing a third distance on the machining path of the wire electrode.
The wire discharge machining method according to the invention further includes making almost constant a temporal change rate of the discharge energy in working the workpiece at at least one of the first step and the fifth step.
Also, the wire discharge machining method according to the invention further includes varying the pulse quiescent time or mean bath voltage as a quadratic function of distance in working the workpiece at at least one of the first step and the fifth step.
Also, the wire discharge machining method according to the invention further includes the decision criterion at the third step of whether or not the discharge frequency is lower than or equal to a predetermined reference value, or the mean bath voltage is Breater than or equal to a predetermined reference value.
The wire discharge machining method according to the invention further includes the decision criterion at the third step of whether or not the discharge frequency is lower than or equal to a predetermined reference value or the stop time of the relative movement is longer than or equal to a reference time, or whether or not the mean bazh voltage is greater than or equal to a predetermined reference value or the stop time of the relative movement is longer than or equal to the reference time.
Also, the wire discharge machining method according to the invention further has different reference values corresponding to the inner corner side and the outer corner side of the corner portion, in which the decision criterion at the third step employs a reference value on the corner side that becomes the product side.
Also, the wire discharge machining method according to the invention further defines the second distance as the distance or simnilar distance from a transit point of the corner portion to a point at which the entire wire electrode fully enters into a working groove after effecting a directional change.
Also, the wire discharge machining method according to the invention further includes at least increasing the set value of wire tension above the set value before working the corner portion (e.g., the first step) or decreasing the set value of pressure or flow of a working fluid jet below the set value before working the corner portion at the second to fourth steps.
A wire discharge machining apparatus according to this invention foi machining a workpiece by supplying a discharge energy into an interpole space between a wire electrode and the workpiece by working power supplying means, and supplying a working fluid into the interpole space through working fluid supplying means, while moving the wire electrode and the workpiece relatively by positioning means comprises control means for controlling the working power supplying means to decrease gradually the discharge energy from a first predetermined distance prior to a corner portion entry on a machining path of the wire electrode, stopping the relative movement between the wire electrode and the workpiece at the corner portion entry, controlling the positioning means to release the stopped relative movement in accordance with a predetermined decision criterion and resume the relative movement, controlling the working power supplying means to decrease he discharge energy for working the workpiece in the corner portion arnn a portion extending a second distance away from the corner portion on the machining path of the wire electrode down to the discharge energy that is reduced at a certain reduction ratio to the discharge energy found at the first predetermined distance prior to the corner portion entry according to at least one of the first condition that is determiried by the corner configuration, such as the radius and angle of the corner portion and the second condition that is determined by the requisition specification such as a desired machining configuration precision, and controlling the working power supplying means to increase gradually the discharge energy after passing the second distance till passing a third distance on the machining path of the wire electrode.
Also, the wire discharge machining apparatus according to the invention further comprises the control means for controlling the working power supplying means to supply the discharge energy on the basis of the pulse quiescent time or mean bath voltage.
Also, the wire discharge machining apparatus according to the invention further comprises the control means for controlling the working power supplying means to decrease or increase gradually the discharge energy by making almost constant a temporal change rate of the discharge energy.
Also, the w4ire discharge machining apparatus according to the invention further comprises the control means for controlling the working power supplying means to decrease or increase gradually the discharge energy by varying the pulse quiescent time or mean bath voltage as a quadratic function of distance.
Also, the wire discharge machining apparatus according to the invention further has the predetermined decision criterion of judging whether or not the discharge frequency is lower than or equal to a certain reference value, or the mean bathvoltage is greater thanorequal to a certain reference value.
Also, the wire discharge machining apparatus according to the invention further has the predetermined decision criterion of whether or not the discharge frequency is lower than or equal to a certain reference value or the stop time of the relative movement is longer than or equal to a reference time, or whether or not the mean bath voltage is greater than or equal to a certain reference value or the stop time of the relative movement is longer than or equal to the reference time.
Also, the wire discharge machining apparatus according to the invention further has different reference values corresponding to the inner corner side and the outer corner side of the corner portion, in which the predetermined decision criterion employs a reference value on the corner side that becomes the product side.
Also, the wire discharge machining apparatus according to the invention further defines the second distance as the distance or similar distance from a transit point of the corner portion to a point at which the entire wire electrode fully enters into a working groove after effecting a directional change.
Also, the wire discharge machining apparatus according to the invention further comprises control means for at least increasing the set value of wire tension above the set value before working the corner portion or decreasing the set value of pressure or flow of a working fluid jet below the set value before working the corner portion ir a course from the point of the corner portion entry at which the relative movement between the wire electrode and the workplece is stopped till passing the second distance away from the corner portion.